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<h1 id="firstHeading" class="firstHeading">Placeholder type specifiers <span class="t-mark-rev t-since-cxx11">(since C++11)</span></h1>
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<tr class="t-nv-h1"><td colspan="5"> Utilities</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> Attributes</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Types</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typedef.html" title="cpp/language/typedef"> <code>typedef</code> declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> Type alias declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
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</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="declarations.html" title="cpp/language/declarations"> Declarations</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Overview </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> declaration syntax</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html#Specifiers" title="cpp/language/declarations"> decl-specifier-seq</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html#Declarators" title="cpp/language/declarations"> declarator</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Specifiers </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="typedef.html" title="cpp/language/typedef"><span class="t-lines"><span>typedef</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="inline.html" title="cpp/language/inline"><span class="t-lines"><span>inline</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> <code>virtual</code> function specifier</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"> <code>explicit</code> function specifier</a>   </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="friend.html" title="cpp/language/friend"><span class="t-lines"><span>friend</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="constexpr.html" title="cpp/language/constexpr"><span class="t-lines"><span>constexpr</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="consteval.html" title="cpp/language/consteval"><span class="t-lines"><span>consteval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="constinit.html" title="cpp/language/constinit"><span class="t-lines"><span>constinit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="storage_duration.html" title="cpp/language/storage duration"> storage class specifiers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="tu_local.html" title="cpp/language/tu local"> translation-unit-local</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="class.html" title="cpp/language/class"><span class="t-lines"><span>class/struct</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="union.html" title="cpp/language/union"><span class="t-lines"><span>union</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="enum_class.html" title="cpp/language/enum"><span class="t-lines"><span>enum</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="decltype.html" title="cpp/language/decltype"><span class="t-lines"><span>decltype</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>auto</span></span></strong></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="alignas.html" title="cpp/language/alignas"><span class="t-lines"><span>alignas</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="cv.html" title="cpp/language/cv"><span class="t-lines"><span>const</span><span>volatile</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier"> elaborated type specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> attributes </a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Declarators </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="reference.html" title="cpp/language/reference"> reference </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="pointer.html" title="cpp/language/pointer"> pointer </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="array.html" title="cpp/language/array"> array </a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Block declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> simple-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> →<a href="structured_binding.html" title="cpp/language/structured binding"> structured binding declaration</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> <code>alias</code> declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace_alias.html" title="cpp/language/namespace alias"> namespace alias definition </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"> using-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html#Using-directives" title="cpp/language/namespace"> using-directive</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_assert.html" title="cpp/language/static assert"> <code>static_assert</code> declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> asm-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="enum_class.html" title="cpp/language/enum"> opaque enum declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Other declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html" title="cpp/language/namespace"> namespace definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> class template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> function template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template"> explicit template instantiation</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> explicit template specialization</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="language_linkage.html" title="cpp/language/language linkage"> linkage specification</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> empty declaration</a> </td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>For variables, specifies that the type of the variable that is being declared will be automatically deduced from its initializer.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>For functions, specifies that the return type will be deduced from its return statements.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>For non-type template parameters, specifies that the type will be deduced from the argument.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">type-constraint</span><span class="t-mark">(optional)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">type-constraint</span><span class="t-mark">(optional)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx14">(since C++14)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">type-constraint</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx20">(since C++20)</span> a <a href="constraints.html#Concepts" title="cpp/language/constraints">concept</a> name, optionally qualified, optionally followed by a template argument list enclosed in <code>&lt;&gt;</code>
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> type is deduced using the rules for <a href="template_argument_deduction.html#Other_contexts" title="cpp/language/template argument deduction">template argument deduction</a>.</div>
<div class="t-li1"><span class="t-li">2)</span> type is <a href="decltype.html" title="cpp/language/decltype"><code>decltype(expr)</code></a>, where <code>expr</code> is the initializer.</div>
<p>The placeholder <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> may be accompanied by modifiers, such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span></span></span>, which will participate in the type deduction. <span class="t-rev-inl t-since-cxx14"><span>The placeholder <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span> must be the sole constituent of the declared type.</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>
</p>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>A placeholder type specifier may appear in the following contexts:
</p>
<ul>
<li> in the type specifier sequence of a variable: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> x <span class="sy1">=</span> expr<span class="sy4">;</span></span></span> as a type specifier. The type is deduced from the initializer.
<br>
If the placeholder type specifier is <code>auto</code> <span class="t-rev-inl t-since-cxx20"><span>or <span class="t-spar">type-constraint</span> <code>auto</code></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>, the variable type is deduced from the initializer using the rules for <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> from a function call (see <a href="template_argument_deduction.html#Other_contexts" title="cpp/language/template argument deduction">template argument deduction — other contexts</a> for details).
<br>
For example, given <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> i <span class="sy1">=</span> expr<span class="sy4">;</span></span></span>, the type of <code>i</code> is exactly the type of the argument <code>u</code> in an imaginary template <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> U<span class="sy3">&amp;</span> u<span class="br0">)</span></span></span> if the function call <span class="t-c"><span class="mw-geshi cpp source-cpp">f<span class="br0">(</span>expr<span class="br0">)</span></span></span> was compiled. Therefore, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span></span></span> may be deduced either as an lvalue reference or rvalue reference according to the initializer, which is used in range-based for loop.
<br>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>If the placeholder type specifier is <code>decltype(auto)</code> <span class="t-rev-inl t-since-cxx20"><span>or <span class="t-spar">type-constraint</span> <code>decltype(auto)</code></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>, the deduced type is <code>decltype(expr)</code>, where <code>expr</code> is the initializer.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<p>If the placeholder type specifier is used to declare multiple variables, the deduced types must match. For example, the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> i <span class="sy1">=</span> <span class="nu0">0</span>, d <span class="sy1">=</span> <span class="nu16">0.0</span><span class="sy4">;</span></span></span> is ill-formed, while the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> i <span class="sy1">=</span> <span class="nu0">0</span>, <span class="sy2">*</span>p <span class="sy1">=</span> <span class="sy3">&amp;</span>i<span class="sy4">;</span></span></span> is well-formed and the <code>auto</code> is deduced as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span>.
</p>
</li><li> in the type-id of a <a href="new.html" title="cpp/language/new">new expression</a>. The type is deduced from the initializer. For <code>new <i>T</i> <i>init</i></code> (where <code><i>T</i></code> contains a placeholder type, <i>init</i> is either a parenthesized initializer or a brace-enclosed initializer list), the type of <code><i>T</i></code> is deduced as if for variable <code><i>x</i></code> in the invented declaration <span class="t-c"><span class="mw-geshi cpp source-cpp">T x init<span class="sy4">;</span></span></span>.

</li><li> <span class="t-mark-rev t-since-cxx14">(since C++14)</span> in the return type of a <a href="function.html" title="cpp/language/function">function</a> or lambda expression: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="sy3">&amp;</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>. The return type is deduced from the operand of its <span class="t-rev-inl t-since-cxx17"><span>non-<a href="if.html#Constexpr_If" title="cpp/language/if">discarded</a></span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> <a href="return.html" title="cpp/language/return">return statement</a>.
<br>
See <a href="function.html#Return_type_deduction" title="cpp/language/function">function — return type deduction</a>.

</li><li> <span class="t-mark-rev t-since-cxx17">(since C++17)</span> in the parameter declaration of a <a href="template_parameters.html#Non-type_template_parameter" title="cpp/language/template parameters">non-type template parameter</a>: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">auto</span> I<span class="sy1">&gt;</span> <span class="kw1">struct</span> A<span class="sy4">;</span></span></span>. Its type is deduced from the corresponding argument.

</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx23"><td>
<p>The <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier may also appear in the simple type specifier of an <a href="explicit_cast.html" title="cpp/language/explicit cast">explicit type conversion</a>: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="br0">(</span>expr<span class="br0">)</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span><span class="br0">{</span>expr<span class="br0">}</span></span></span>. Its type is deduced from the expression.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx23">(since C++23)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>Furthermore, <code>auto</code> <span class="t-rev-inl t-since-cxx20"><span>and <span class="t-spar">type-constraint</span> <code>auto</code></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span> can appear in:
</p>
<ul><li> the parameter declaration of a <a href="lambda.html" title="cpp/language/lambda">lambda expression</a>: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;&amp;</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span></span></span>. Such lambda expression is a <i>generic lambda</i>.
</li><li> <span class="t-mark-rev t-since-cxx20">(since C++20)</span> a <a href="function.html#Parameter_list" title="cpp/language/function">function parameter declaration</a>: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span><span class="sy4">;</span></span></span>. The function declaration introduces an <a href="function_template.html#Abbreviated_function_template" title="cpp/language/function template">abbreviated function template</a>.
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<h4> <span class="mw-headline" id="Type_constraint">Type constraint</span></h4>
<p>If <span class="t-spar">type-constraint</span> is present, let <code>T</code> be the type deduced for the placeholder, the <span class="t-spar">type-constraint</span> introduces a <a href="constraints.html" title="cpp/language/constraints">constraint expression</a> as follows:
</p>
<ul><li> If <span class="t-spar">type-constraint</span> is <code>Concept&lt;A<sub>1</sub>, ..., A<sub>n</sub>&gt;</code>, then the constraint expression is <code>Concept&lt;T, A<sub>1</sub>, ..., A<sub>n</sub>&gt;</code>;
</li><li> otherwise (<span class="t-spar">type-constraint</span> is <code>Concept</code> without an argument list), the constraint expression is <code>Concept&lt;T&gt;</code>.
</li></ul>
<p>Deduction fails if the constraint expression is invalid or returns <code>false</code>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>Until C++11, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> had the semantic of a <a href="storage_duration.html" title="cpp/language/storage duration">storage duration specifier</a>.
</p><p>Mixing <code>auto</code> variables and functions in one declaration, as in <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <span class="kw4">int</span>, i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span></span></span> is not allowed. 
</p><p>The <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier may also be used with a function declarator that is followed by a trailing return type, in which case the declared return type is that trailing return type (which may again be a placeholder type).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <span class="kw4">int</span><span class="sy4">;</span> <span class="co1">// declares p as pointer to function returning int</span>
<span class="kw4">auto</span> <span class="br0">(</span><span class="sy2">*</span>q<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <span class="kw4">auto</span> <span class="sy1">=</span> p<span class="sy4">;</span> <span class="co1">// declares q as pointer to function returning T</span>
                         <span class="co1">// where T is deduced from the type of p</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>The <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier may also be used in a <a href="structured_binding.html" title="cpp/language/structured binding">structured binding</a> declaration.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-concepts_ts"><td>
<p>The <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> keyword may also be used in a nested-name-specifier. A nested-name-specifier of the form <span class="t-c"><span class="mw-geshi cpp source-cpp">auto<span class="sy4">::</span></span></span> is a placeholder that is replaced by a class or enumeration type following the rules for <a href="../experimental/constraints.html" title="cpp/experimental/constraints">constrained type</a> placeholder deduction.
</p>
</td>
<td><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;utility&gt;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">auto</span> add<span class="br0">(</span>T t, U u<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t <span class="sy2">+</span> u<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// the return type is the type of operator+(T, U)</span>
 
<span class="co1">// perfect forwarding of a function call must use decltype(auto)</span>
<span class="co1">// in case the function it calls returns by reference</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> F, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> PerfectForward<span class="br0">(</span>F fun, Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span><span class="br0">)</span> 
<span class="br0">{</span> 
    <span class="kw1">return</span> fun<span class="br0">(</span><a href="../utility/forward.html"><span class="kw1120">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>Args<span class="sy1">&gt;</span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span> 
<span class="br0">}</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">auto</span> n<span class="sy1">&gt;</span> <span class="co1">// C++17 auto parameter declaration</span>
<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> <a href="../utility/pair.html"><span class="kw1099">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span>n<span class="br0">)</span>, decltype<span class="br0">(</span>n<span class="br0">)</span><span class="sy1">&gt;</span> <span class="co1">// auto can't deduce from brace-init-list</span>
<span class="br0">{</span>
    <span class="kw1">return</span> <span class="br0">{</span>n, n<span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">auto</span> a <span class="sy1">=</span> <span class="nu0">1</span> <span class="sy2">+</span> <span class="nu0">2</span><span class="sy4">;</span>          <span class="co1">// type of a is int</span>
    <span class="kw4">auto</span> b <span class="sy1">=</span> add<span class="br0">(</span><span class="nu0">1</span>, <span class="nu16">1.2</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// type of b is double</span>
    static_assert<span class="br0">(</span><a href="../types/is_same.html"><span class="kw591">std::<span class="me2">is_same_v</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span>a<span class="br0">)</span>, <span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>
    static_assert<span class="br0">(</span><a href="../types/is_same.html"><span class="kw591">std::<span class="me2">is_same_v</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span>b<span class="br0">)</span>, <span class="kw4">double</span><span class="sy1">&gt;</span><span class="br0">)</span><span class="sy4">;</span>
 
    <span class="kw4">auto</span> c0 <span class="sy1">=</span> a<span class="sy4">;</span>             <span class="co1">// type of c0 is int, holding a copy of a</span>
    decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> c1 <span class="sy1">=</span> a<span class="sy4">;</span>   <span class="co1">// type of c1 is int, holding a copy of a</span>
    decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> c2 <span class="sy1">=</span> <span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// type of c2 is int&amp;, an alias of a</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"before modification through c2, a = "</span> <span class="sy1">&lt;&lt;</span> a <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="sy2">++</span>c2<span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">" after modification through c2, a = "</span> <span class="sy1">&lt;&lt;</span> a <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="kw4">auto</span> <span class="br0">[</span>v, w<span class="br0">]</span> <span class="sy1">=</span> f<span class="sy1">&lt;</span><span class="nu0">0</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">//structured binding declaration</span>
 
    <span class="kw4">auto</span> d <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK: type of d is std::initializer_list&lt;int&gt;</span>
    <span class="kw4">auto</span> n <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>    <span class="co1">// OK: type of n is std::initializer_list&lt;int&gt;</span>
<span class="co1">//  auto e{1, 2};    // Error as of DR n3922, std::initializer_list&lt;int&gt; before</span>
    <span class="kw4">auto</span> m<span class="br0">{</span><span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>       <span class="co1">// OK: type of m is int as of DR n3922, initializer_list&lt;int&gt; before</span>
<span class="co1">//  decltype(auto) z = { 1, 2 } // Error: {1, 2} is not an expression</span>
 
    <span class="co1">// auto is commonly used for unnamed types such as the types of lambda expressions</span>
    <span class="kw4">auto</span> lambda <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x <span class="sy2">+</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">//  auto int x; // valid C++98, error as of C++11</span>
<span class="co1">//  auto x;     // valid C, error in C++</span>
 
    <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span>...<span class="br0">)</span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>c0, c1, v, w, d, n, m, lambda<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// suppresses "unused variable" warnings</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">before modification through c2, a = 3
 after modification through c2, a = 4</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1265.html">CWG 1265</a>
</td>
<td> C++11
</td>
<td> the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier could be used to declare a function with a trailing<br>return type and define a variable in one declaration statement
</td>
<td> prohibited
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1346.html">CWG 1346</a>
</td>
<td> C++11
</td>
<td> a parenthesized expression list could not be assigned to an auto variable
</td>
<td> allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1347.html">CWG 1347</a>
</td>
<td> C++11
</td>
<td> an declaration with the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier could define two variables<br>with types <code>T</code> and <code>std::initializer_list&lt;T&gt;</code> respectively
</td>
<td> prohibited
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1852.html">CWG 1852</a>
</td>
<td> C++14
</td>
<td> the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> specifier in <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span></span></span> was also a placeholder
</td>
<td> not a placeholder in this case
</td></tr></table>

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